A door of an automobile is produced by fitting a door inner panel, a door outer panel and the like together. A widow, a window drive device, an audio speaker, a handle and the like are to be attached to the door of an automobile. For set-in of these components, a space must be made between the door inner panel and the door outer panel. In order to make the space, for example, vertical walls are made in the door inner panel. Also, when the door is closed, the inside of the automobile must be sealed by the door. In order to seal the inside of the automobile, a stair is made in the door inner panel. The stair of the vertical walls is caused to face a pillar and the like, and thereby, the inside of the automobile can be sealed tighter.
A door inner panel used for an automobile side door or the like is produced by press working of a steel plate. Such a door inner panel generally has a complicated shape. Accordingly, in producing such a door inner panel, it is necessary to deform a steel plate largely. In this case, the door inner panel after the press working may have cracks, wrinkles and the like. Especially in a case of production of the door inner panel with a stair and adjacent vertical walls, the door inner panel after the press working is likely to have cracks, wrinkles and the like.
In order to avoid the problem, typically, a highly workable steel plate is used as the material (blank) for such a door inner panel. A highly workable steel plate is highly ductile, and thereby, even when the highly workable steel plate is deformed, cracks, wrinkles and the like are unlikely to occur. As another measurement to avoid the problem, hot stamping is adopted to production of a door inner panel. In hot stamping, a material is heated and pressed. The workability of the material is heightened by heat, and the door inner panel after the press working is unlikely to have cracks, wrinkles and the like.
Methods for manufacturing a panel-like formed product including a door inner panel are disclosed in WO 2014/084151 (Patent Literature 1) and Japanese Patent Application Publication 2010-149184 (Patent Literature 2).
A panel-like formed product manufacturing method disclosed in Patent Literature 1 includes a vertical wall forming step and a flange forming step. In the vertical wall forming step, a blank is pressed with a punch and a die, and the vertical walls are wholly formed. In the flange forming step after the vertical wall forming step, while the vertical walls are caught between the punch and the die, the top board is pinched between a pad and a punch, and thereby, a flange is formed. Patent Literature 1 states as follows. In the vertical wall forming step, the ends of the blank are not tied down, and therefore, the vertical walls are unlikely to spring back. In the flange forming step, the top board and the vertical walls are tied down by the die, and therefore, the flange can be formed with high accuracy.
In a panel-like formed product manufacturing method disclosed in Patent Literature 2, vertical walls are formed by hot stamping while a portion of a blank to be formed into a top board is pinched between a pad and punch. When press working is applied to the blank, the ends of the blank do not make contact with the die, and therefore, the ends of the blank are not quenched. Patent Literature 2 states that this method reduces the risk of causing cracks in the end portions of the pressed product.